dt-bindings: mfd: Convert STMPE to YAML schema

dt-bindings: mfd: Convert STMPE to YAML schema

This converts the STMPE MFD device tree bindings to the YAML
schema.

Reference the existing schema for the ADC, just define the
other subnode schemas directly in the MFD schema.

Add two examples so we have examples covering both the simple
GPIO expander and the more complex with ADC and touchscreen.

Some in-tree users do not follow the naming conventions for nodes
so these DTS files need to be augmented to use proper node names
like "adc", "pwm", "gpio", "keyboard-controller" etc before the
bindings take effect on them.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230709-stmpe-dt-bindings-v5-1-34a3d6ee1e57@linaro.org
Signed-off-by: Lee Jones <lee@kernel.org>